The present invention relates to a method of magnetic resonance imaging for imaging the internal structure of substances utilizing nuclear magnetic resonance (NMR) or electron spin resonance (ESR). In one aspect of the invention, it relates more particularly to slice imaging which produces a plurality of images by means of multiple echoes. In another aspect of the invention, it relates more particularly to slice imaging which utilizes chemical shifts which differ depending on the chemical status of the particular substance.
FIG. 1 shows an example of a conventional NMR imaging apparatus. This apparatus comprises a static magnetic field generator 51 for applying a static magnetic field to a sample 50 to be examined, such as a human body, an RF (radio frequency) coil 55 for supplying an RF energy to the sample 50 and receiving an NMR signal from the sample 50, and gradient magnetic field coils 60, 62 and 64 for applying gradient magnetic fields to the sample 50.
The RF coil 55 irradiates RF pulses which are generated at a transmitter 58, and transmitted via the transmit/receive switch 57 and a matching unit 56. NMR signals from the sample 50 are received by the RF coil 55, and transmitted via the matching unit 56 and the transmit/receive switch 57 to a receiver 59.
The gradient magnetic field coils 60, 62 and 64 are respectively connected to the gradient magnetic field power supplies 61, 63 and 65 for generating gradient magnetic fields in the X-axis, Y-axis and Z-axis directions respectively. Here, the magnetic field generated by the Z-axis gradient magnetic field coil 64 is the slicing magnetic field Gs for designating the slice plane. The magnetic field generated by the X-axis gradient magnetic-field coil 60 is the signal reading magnetic field Gp for collecting signals. The magnetic field generated by the Y-axis gradient magnetic field coil 62 is the phase encoding magnetic field Gen.
A computer 52 generates control data for the imaging apparatus, and generates images in accordance with the NMR signals from the receiver 59 by means of two-dimensional Fourier conversion. The computer 52 is associated with a console unit 53 for manual input of commands and data and a display unit 54 for displaying the generated images and other data.
A sequence controller 67 controls various operations of the entire apparatus, such as irradiation of the RF pulses, reception of the NMR signals, generation of the gradient magnetic fields Gs, Gp and Gen, and operation of a diagnosis table driving unit 66.
FIG. 2 is a timing chart for explaining a prior art multi-echo imaging method.
In this imaging method, a slicing magnetic field Gs (B1) is applied to designate the slice plane at the same time as the irradiation of a 90.degree. RF pulse (A1) for generating an NMR signal. After that, a slicing magnetic field Gs (B2) of the opposite polarity is applied to rectify the disorder of the phase of the nuclear spins. The areas B1 and B2 shown by hatching in FIG. 2 are set to be equal to each other.
Subsequently, a reading magnetic field Gp (D1) and a phase encoding magnetic field Gen (C1) are applied. The reading magnetic field Gp (D1) is applied for causing disorder in the phase of the NMR signals in the direction of reading.
Subsequently, a 180.degree. RF pulse (A2) and another slicing magnetic field Gs (B3) are applied .tau..sub.1 after the peak of the 90.degree. RF pulse. This will produce a spin echo signal (E1) .tau..sub.1 after the peak of the 180.degree. RF pulse (A2). The reading magnetic field Gp is applied in such a way that the area of D2 equals the area of D1.
In order to read the spin echo signal (E1), another reading magnetic field Gp (D3) is applied immediately following the reading magnetic field (D2) without interruption.
In order to produce a second spin echo signal (E2), another 180.degree. RF pulse (A3) having a peak .tau..sub.2 after the generation of the spin echo signal and another slicing magnetic field Gs (B4) are applied. The reading magnetic field Gp (D4) is applied in such a way that the area of D4 and the area of D3 are equal to each other. Another reading magnetic field Gp (D5) is applied without interruption. A second spin echo signal (E2) is then obtained.
In order to obtain a third spin echo signal (E3), in the same manner as described above in connection with the generation of the spin echo (E2), a reading magnetic field Gp (D6) is applied in such a way that the area of D6 equals the area of D5. Similarly, another reading magnetic field Gp (D8) is applied in such a way that the area of D8 equals the area of D7.
The above-described sequence of operations yields a series of signals for one direction of reading. In order to obtain an image of a predetermined matrix size, the phase encode amount is varied, i.e., the amplitude of the phase encoding magnetic field Gen (C1) is varied by a predetermined amount between succesive sequences of operations, and series of echo signals (E1), (E2), (E3) and (E4) are collected a number of times corresponding to the matrix size. The computer 52 reconstructs a multi-echo image from the echo signals (E1), (E2), (E3) and (E4) thus obtained.
A problem associated with the method of FIG. 2 is that it is possible that an NMR signal generated because of the imperfection of the 180.degree. RF pulse, is generated overlapping the spin echo signal that is to be detected. Particularly, where .tau..sub.1 =.tau..sub.2 =.tau..sub.3 =.tau..sub.4, after the third spin echo signal, a stimulated echo signal due to the imperfection of the 180.degree. RF pulse is generated at the same position as the spin echo signal so a false image called "artifact" is generated in the image of the third spin echo signal and the subsequent images. This seriously degrades the quality of the image.
FIG. 3 is a timing chart for explaining another conventional chemical shift NMR imaging method that is shown in Denshi Tsushin Gakkai Gijutsu Hokoku (Technical Reports of the Institute of Electronics and Communication Engineers of Japan) MBE 85-9.
Generally speaking, if none of the X-axis, Y-axis and Z-axis gradient magnetic fields are applied, when a 90.degree. RF pulse is irradiated and a 180.degree. RF pulse is irradiated a predetermined time after the irradiation of the 90.degree. RF pulse, then an echo signal is generated upon expiration of the same time as the above-mentioned predetermined time after the irradiation of the 180.degree. RF pulse. This echo signal is called an echo signal due to an RF pulse.
If a signal reading magnetic field Gp is applied for a predetermined time period between the irradiation of a 90.degree. RF pulse, and the irradiation of a 180.degree. RF pulse, and another signal reading magnetic field Gp of the same intensity is applied after the irradiation of the 180.degree. RF pulse, then a spin echo signal is generated upon expiration of the above-mentioned predetermined time after the commencement of the application of the later signal reading magnetic field Gp. This spin echo signal is called a spin echo signal due to a gradient magnetic field.
Where there are two chemical shifts, there are two different resonant frequencies. This means that the phase of the spin varies at a rate corresponding to the difference between the resonant frequencies. The phases of the spins are in alignment when an echo due to the RF pulse is generated, but in the case of spin echoes due to a gradient magnetic field only such phases that are due to the application of the gradient magnetic field can be aligned and other phases remain out of order.
If this characteristic is utilized for staggering the time point of generation of the spin echo due to the RF pulse and the time point of generation of the spin echo due to the gradient magnetic field, then it is possible to vary the phases of the signals having different chemical shifts, and a phase different proportional to the time difference can be generated.
The above expression "two chemical shifts" means presence of two chemical shifts in the spins which are the object of the imaging of the slice. For instance, when an NMR imaging method directed to protons is used for imaging a human body, two types of protons, i.e., protons mainly of water and protons of fat are imaged. Protons of water and protons of fat have slightly different sensitivity to a static magnetic field. Even when protons of water and protons of fat are placed in the same static magnetic field, the static magnetic field they actually sense differs slightly. The difference in the sensitivity is in the order of 3.5 ppm. In a magnetic field of 1.5 T, the difference in frequency is given by: EQU 1.5(T).times.3.5(ppm).times.10.sup.-6 .times.42.6.times.10.sup.6 (Hz/T)=220 (Hz)
This value of difference depends on the physical property of the spins which are the object of the imaging. Because of this property, the number of chemical shifts is determined.
Where there are two or more chemical shifts, the phases of the spin echo due to the RF pulse and the spin echo due to the gradient magnetic field can be varied, and the variation in the phases are proportional to the respective chemical shift amounts. Accordingly, in order to separate all the chemical shifts, the time period that is taken until the spin echo due to the gradient magnetic field is generated must have a different value for each of the chemical shifts.
The conventional method of separation will now be described with reference to FIG. 3.
First, a 90.degree. RF pulse F.sub.11 is irradiated, and by application of a waveform K.sub.11 of the slicing magnetic field Gs, the slice position is selected, and the disorder of the phases of the nuclear spins are rectified by a waveform K.sub.21 of the polarity opposite to that of the waveform K.sub.11. Upon expiration of .tau..sub.11 after the irradiation of the 90.degree. RF pulse, a 180.degree. RF pulse F.sub.21 is irradiated to generate in combination with a slicing magnetic field Gs (K.sub.31), a spin echo J.sub.11.
In an ordinary imaging, the timing of output of the waveform I.sub.2 is so adjusted that the time period of application from the rising edge of the waveform I.sub.2 until the peak of the spin echo J.sub.11 is equal to t.sub., which is the time period of application of the waveform I.sub.1 of the reading magnetic field Gp so that the peak of the spin echo takes place upon expiration of .tau..sub.21 (.tau..sub.21 =.tau..sub.11) from the 180.degree. RF pulse F.sub.21. The intensities of the magnetic fields of the waveforms I.sub.1 and I.sub.2 are equal. In this condition, the phase variance due to the chemical shift and the reading magnetic field Gp are canceled at the time point of occurrence of the peak of the spin echo signal J.sub.11. Where there are two or more chemical shifts, all the signals are obtained with the same phase at the same time.
To separate the chemical shifts, the time period .tau..sub.12 up to the irradiation of the 180.degree. RF pulse F.sub.22 is varied so that the phase variance due to the chemical shift and the phase variance due to the signal reading magnetic field Gp are staggered. In this case, it is necessary that the slicing magnetic field Gs be shifted to coincide with the irradiation of the 180.degree. RF pulse. For this reason, even if the time period .tau..sub.12 up to the 180.degree. RF pulse F.sub.22 is changed the time point at which the peak of the spin echo signal J.sub.12 occurs is fixed at t.sub.1 after the commencement of the application of the second signal reading magnetic field Gp. Accordingly, the phase variances due to the signal reading magnetic fields Gp is canceled. However, the phase variance amount (proportional to .tau..sub.12) between the 90.degree. RF pulse and the 180.degree. RF pulse due to the difference in the chemical shift and the phase variance amount (proportional to .tau. .sub.22) after the 180.degree. RF pulse are not canceled because .tau..sub.12 .noteq..tau..sub.22. For instance, assume that there are two chemical shifts and the difference in frequency between the chemical shifts is .DELTA. f. During the time period until the occurrence of the peak of the spin echo signal J.sub.12, the phase variance amount 2.pi..multidot.(.DELTA.f).multidot..tau..sub.12 due to the time period .tau..sub.12 and the phase ariance amount 2.pi..multidot.(.DELTA.f).multidot..tau..sub.22 due to the time period .tau..sub.22 differ from each other by:
ti 2.pi..multidot.(.DELTA.f).multidot..tau..sub.12 -2.pi..multidot.(.DELTA.f).multidot..tau..sub.22 =2.pi..multidot.(.DELTA.f).multidot.(.tau..sub.12 -.tau..sub.22)
By adjusting the timing of irradiation of the 180.degree. RF pulse F.sub.22 so that 2.pi..multidot.(.DELTA.f).multidot.(.tau..sub.12 -.tau..sub.22)=.+-..pi., signals of all the chemical shifts (in the case under consideration, two chemical shifts) can be extracted with different phases.
The phase of the chemical shifts of the spin echo signals J.sub.11 and J.sub.12 are opposite to each other so the signs of the signals are opposite. Accordingly, if the sums of the spin echo signals J.sub.11 and J.sub.12 are determined, there would not be signals of chemical shift of which the phase variance amount has been canceled. If the differences are determined, only such signals of the chemical shifts of which the phase variance amount has not been canceled are obtained. When the signals of the sums and the differences that have thus been obtained are utilized, various images of the respective chemical shifts are obtained independently.
However, in the method of the chemical shift imaging of FIG. 3, it is necssary to synchronize the 180.degree. RF pulse F.sub.21 for signal collection and the waveform of the slicing magnetic field Gs (K32) and shift them by the same time length, resulting in control difficulties.